The present invention relates to a method for reducing and/or preventing the fouling of process equipment in the preparation of O,O-di(C.sub.1 -C.sub.8 alkyl) phosphorochloridothioates. It is particularly applicable to process equipment involving the separation and purification of O,O-di(C.sub.1 -C.sub.8 alkyl) phosphorochloridothioates from a mixture thereof with certain impurities. Preferably, the improved method comprises adding to the crude dialkyl phosphorochloridothioate the condensation product of a phenol and preferably a high molecular weight alkylphenol, an aldehyde and an amine containing at least one H--N&lt;group. The O,O-dialkyl phosphorochloridothioates are valuable intermediates, for instance, in the preparation of lubricant additives and insecticides. Particularly, O,O-diethyl thiophosphoryl chloride is an intermediate in the synthesis of an insecticide known as parathion, and O,O-dimethyl thiophosphoryl chloride is an intermediate in the synthesis of the insecticide called methyl parathion. Such are also useful in the manufacture of diazinon, chlorpyrifus, fensulfothion and the like.
Several methods have been used for the synthesis of the esters of phosphorochloridothioic acid including one-step and two-step methods. In the one-step process, phosphorus pentasulfide, alcohol and chlorine are reacted to prepare the ester corresponding to the alcohol and then the solvent is removed and the product separated. Typical prior art patents disclosing a one-step process include U.S. Pat. Nos. 3,356,774 and 3,502,750. U.S. Pat. No. 3,356,774 discloses reacting a phosphorus pentasulfide suspension in an inert solvent at a temperature within the range of about 0.degree. C. to about 150.degree. C. with chlorine and an alcohol having 1 to 6 carbon atoms by introducing a stream of chlorine into said suspension and adding dropwise concurrently therewith the alcohol, allowing the chlorinating reaction to proceed to completion, expelling the solvent from the resulting reaction mixture and isolating O,O-dialkylthionophosphoric acid chloride by distillation. U.S. Pat. No. 3,502,750 discloses preparing lower alkyl esters of phosphorochloridothioic acid by reacting chlorine with a lower alkyl ester of dithiophosphoric acid and freeing the product of sulfur monochloride by reaction with hydrogen sulfide, preferably formed during the production of the dithiophosphoric acid ester by reaction of a lower alkanol with phosphorus pentasulfide.
In the two-step process, the first process step reacts phosphorus pentasulfide with an alcohol, such as ethanol, so as to form O,O-diethyl dithiophosphoric acid and hydrogen sulfide, and in a second process step the isolated O,O-diethyl dithiophosphoric acid is chlorinated in an appropriate solvent with chlorine gas, resulting in the formation of O,O-diethyl thiophosphoric acid chloride. Examples of prior art patents disclosing a two-step process include U.S. Pat. Nos. 3,836,610 and 3,856,898. In U.S. Pat. No. 3,836,610, the reaction mixture is chlorinated and then established and maintained at a temperature in the range of 85.degree. C.-110.degree. C. until it is substantially free of sulfur monochloride and the relatively thermal unstable sulfur that forms becomes more thermally stable so that the product dialkyl thiophosphoryl chloride can be readily and safely removed from the mixture thereof with sulfur by distillation. U.S. Pat. No. 3,856,898 discloses a process for treating a mixture comprising O,O-di(C.sub.1 -C.sub.8 alkyl) phosphorochloridothioate and amphorous sulfur at a concentration up to about one-third of the weight of the phosphorochloridothioate. In this process, the mixture is established in a first temperature range in which substantially all of the sulfur can go into solution without substantial decomposition of the phosphorochloridothioate, and maintained in that range until substantially all of the sulfur does go into solution. The resulting solution is established in a temperature range in which dissolved sulfur crystallizes, and is maintained in that range until sulfur crystallization is substantially complete. The crystallized sulfur then is separated by settlement (filtration, decantation, centrifugation, or the like) from the mother liquor. In one embodiment, the mother liquor, composed of the phosphorochloridothioate dissolved in a solvent, is treated by a procedure which includes distillation to obtain O,O-di(C.sub.1 -C.sub.8 alkyl) phosphorochloridothioate. Other prior art patents which disclose processes for preparing O,O-dialkyl phosphorochloridothioates include U.S. Pat. No. 3,897,523 which teaches a purification process in which the crude dialkyl phosphorochloridothioate is vaporized in a film evaporator, the vapor is condensed, washed with water at 10.degree. C. to 60.degree. C., the organic and aqueous phases separated and the organic phase vacuum dried; and U.S. Pat. No. 4,025,586, which discloses distilling the product dialkyl phosphorochloridothioate and water washing the distillation residue to hydrolyze impurities. The washed residue is then dried and recycled to the chlorination step. U.S. Pat. No. 3,089,890, teaches treating a distilled crude phosphorochloridothioate with water, separating the organic phase and drying to upgrade the crude and recover substantially contaminant-free phosphorochloridothioate. Most recently, U.S. Pat. No. 4,159,289 teaches a process for removal of sulfur impurities from phosphorochloridothioates by distillation in the presence of a naphthalenic liquid hydrocarbon sulfur solubilizing or suspending agent.
Conventionally, the alkyl groups in the dialkyl phosphororchloridothioates have from 1 to 8 carbon atoms and are generally selected from methyl, ethyl, isopropyl, butyl, sec.butyl, t-butyl, and the like, up through n-octyl and isomers thereof. In each of these conventional one-step and two-step processes, however, impurities, such as phosphates are produced which causes severe fouling of the process equipment during separation and purification of product dialkyl phosphorochloridothioates. Although these various processes differ somewhat as to the precise manner in which product dialkyl phosphororochloridothioates are produced, those processes which involve the heating of a crude feed stock to a high temperature and the passage of such heated stock through a distillation column to separate and recover product dialkyl phosphorochloridothioate from the crude feed stock almost always result in the formation of some undesirable materials, believed principally to comprise oxygenated phosphorous compound impurities produced during production of the phosphorochloridothioates as by-products or from the thermal degradation of the desired dialkyl phosphorochloridothioates during purification, along with impurities other than the aforementioned oxygenated phosphorus type impurities such as iron and/or sulfur or iron and sulfur containing compounds. These impurities solidify in and plug the distillation columns, and adhere to the walls of the tubes in the column reboiler sections of the columns as the impurities containing crude feed stock passes through or around the tubes. This lowers the efficiency, principally by impeding the flow of the feed stock therethrough, and the transfer of heat to or from such stock. After enough material has accumulated on the various parts on the reboiler units, usually the tube portions thereof, to lower efficiency substantially, the unit must be dismantled, cleaned and reassembled. Of course, such cleaning operations are not only tedious and costly, but result in a large proportion of "down-time" during which the unit is not functioning. Distillation alone cannot adequately remove these impurities.
The deposit formations resulting from the fouling phenomenon consist of a tacky, water soluble tar material believed to be composed principally of polyphosphates produced during the production of the phosphorochloridothioates as by-products aforediscussed and/or from the desired product dialkyl phosphorochloridothioates which may be thermally degraded over time in the purification equipment to produce additional by-products of the same sort. Specific impurities which are formed include diethylchlorophosphate, triethylthiophosphate, ethyldichlorophosphate and ethyldichlorothiophosphate. The thermal degradation of diethyl phosphorochloridothioate is illustrative of what may occur during processing operations. A sample of diethyl phosphorochloridothioate was found to contain 88.4 area percent diethyl phosphorochloridothioate and 0.48 area percent diethyl phosphoryl chloride by vapor phase chromatography. The sample was split in two parts and one was purged with air while the other was purged with nitrogen. The samples were heated at 140.degree. C. for 4 hours. Samples taken after one-half hour and 3 hours were analyzed with the results shown below:
______________________________________ Increase of Oxygenated Phosphorus Impuritites with Time at 140.degree. C. Time, hrs. 0 1/2 3 ______________________________________ Air Purge Diethylphosphorochlorido- thioate (Area %) 88.4 87.3 78.9 Diethylphosphoryl chloride (Area %) 0.48 0.59 2.4 N.sub.2 Purge Diethylphoschlorochlorido- thioate (Area %) 88.4 87.1 71.4 Diethylphosphoryl chloride (Area %) 0.48 0.68 2.3 ______________________________________
From the above data, it is clear that the impurity has increased fivefold while where was a 9-20% decrease in desired product. Such high impurity levels are not only undesirable for the aforediscussed fouling problems which they create in the distillation equipment, but are also undesirable because the end-product insecticides have been registered with governmental agencies as having been tested and found safe and effective with certain impurities at not greater than certain concentrations. Therefore, it is critical not only to prevent the fouling of process equipment by the impurities but also to maintain impurity identities and levels at or below those allowed in governmental registrations and as stated on label certifications for the end product.
Without limiting the invention in any manner and without advocating any particular mechanism or theory of action, it is believed that degradation of dialkyl phosphorochloridothioates could possibly take place according to the following chemical reaction scheme: ##STR1## where R.sub.1 and R.sub.2 can be the same or different C.sub.1-8 alkyl groups, the temperature ranges from 100.degree.-150.degree. C., and the necessary contact with ionic species is provided for a time sufficient to facilitate the degradation reaction. Further, it is believed that the presence of other impurities, such as sulfur, for example, in the form either of free sulfur or in some other form such as sulfur monochloride, formed as a by-product during the reaction; and/or iron which may be present in the crude feedstock or originate from deterioration of the metal in the process equipment may also either constitute a portion of the fouling deposits or alternatively exert a catalytic effect on and promote or increase the rate of phosphorus impurities formation. While fouling sometimes may be controlled by careful processing to obtain high purity crude feedstock by excluding impurities from the crude feed prior to separation and purification of product dialkyl phosphorochloridothioate therefrom, this is not altogether possible nor economically feasible in plant scale operation on a regular continuing basis. One method for removing dialkyl phosphorochloridothioates impurities from a crude feed stock containing same is disclosed in U.S. Appl. Ser. No. 073,684, filed Sept. 10, 1979 which teaches treating the crude feed stock with an alcohol having from 2 to 30 carbon atoms so that the impurities and the alcohol from a reaction product having sufficiently different physical properties to promote the separation of product dialkyl phosphorochloridothioates from the resulting mixture.